Stick welding electrodes are manufactured by cutting the welding wire into lengths of less than about 16 inches and extruding a pliable mass with a settable binder around a straight section of welding wire. The coating includes particles, such as particles for fluxing system, particles for arc stabilizing and alloying particles. The binder is normally sodium silicate to create a viscosity allowing extrusion of the mass onto the welding wire. The extruded mass is then heated to set the binder into a generally hard coating on the outer surface of the electrode. The heating or drying operation is performed on electrodes moving by a conveyor in a side-by-side relationship after the strike end has been ground and the hold end has been brushed to expose the wire subsequently used in the welding operation. As the electrodes move transversely, they are heated by a convection furnace to dry the coating. This drying operation is costly and time consuming and requires adjustment and control to assure proper hardening and the desired moisture content of the dry coating. In cellulose type coatings, as commonly used in pipe welding, it is desirable to have a high moisture content in the coating. The moisture of the coating normally starts with about 20%. By using the convection drying process, obtaining high moisture content in a cellulose coating is quite difficult due to the formation of an outer skin which is hardened first in the external drying process. In other electrodes, the moisture content is to be maintained at a level less than 1% and preferably less than 0.5% of the total weight of the electrode. In a cellulose coating, the outer skin effect created by convection heating contributes to the tendency of the electrode to form blisters at the end of the electrode as it is melted during the welding process. Such blisters cause erratic arc directions and uneven formation of the weld metal during the welding process. In this prior art drying, the coating is often hardened before the internal welding wire is thoroughly heated. Since the hardened coating and weld metal have different thermal expansion coefficients, the hardened coating is cracked as the wire temperature increases as the electrode moves through the convection furnace. Conventional dryers harden the coating first and it loses elasticity. The core wire is then heated by conduction through the coating. The coating and wire eventually reach or approach the same temperature, but at significantly different times. Consequently, their different coefficients of thermal expansion causes the coating to be pulled apart and creates minute circumferential cracks in the coating. Circumferential cracking is a particular problem with certain electrodes, such as electrodes with a substantial amount of steel powder in the coating. The cost, difficult controllability, and early hardening of the coating during the existing drying process are disadvantages to which the present invention is directed.
In accordance with the present invention, after the pliable mass of coating material has been extruded over the welding wire and the strike and hold ends have formed, the electrodes are moved in succession through an induction heating device. This device hardens the coating using an induction heating process used by itself or in combination with convection and/or radiant heating, before, after or during the induction heating process. By using the present invention, moisture of the coating is controlled and the drying process is extremely repetitive, clean, highly efficient and environmentally friendly. The drying device and process prevents cracks that contribute to a burn-off and forms no outer hardened skin to cause blistering of the coating as it progresses toward the welding arc. Since there is no outer skin caused by the drying process, cellulose coatings can have increased moisture content in the range of 2-5% or even higher. This improves the porosity of the weld metal and does not cause blisters normally associated with high moisture content in a cellulose coating for a stick electrode. Heating rate and time is accurately controlled using induction heating and which facilitates high moisture content for cellulose coatings. Indeed, the moisture content can be reduced from a high starting content, such as 20% of the coating, to a high level of 2-5% of the electrode coating weight. The use of induction heating raises the temperature of the center welding wire immediately. Any magnetic particles in the coating are also heated. By immediately starting the heating process in the wire, instead of externally of the coating, the coating is dried from the inside toward the outside. This prevents the formation of an outer hard impermeable type of skin as resulting from convection and/or radiant heating as used in the past. The circumferential cracking experienced during conventional drying of the coating and caused by unequal thermal expansion of the coating and core wire is avoided in induction heating. When using induction heating, the wire is heated by inducing a voltage difference in the wire causing current flow through the outer portion of the wire so the wire and coating are heated simultaneously. Induction heating of the core wire is precisely controlled by the power and frequency of the induction heating process. The heating time is accurately controlled, since it can be turned on and off by merely discontinuing the heating current. Convection heating, to the contrary, can not be accurately controlled in this manner. Due to the ability to control the rate and heating time, the heating of the core and coating can be coordinated to prevent any temperature induced circumferential cracking as in the prior drying devices.
Induction heating overcomes the blistering effect experienced with cellulose electrodes. Since the coating during the drying operation does not form an outer impervious skin, the skin can not trap steam in small pockets to form undesired surface blisters. Thus, by using the present invention, a cellulosic or cellulose type of coating can have a high moisture content necessary for porosity without the deleterious effect of surface blistering. This is a substantial advance in the drying process for a coating using cellulose. In the past, a convection furnace caused skin on a cellulose coating because the coating was dried by hot air only. The use of induction heating in combination with convection and/or radiant heating does not create this outer skin because the coating is dried by conductive heating from the inner wire. This internal heating of the welding wire is performed at precisely controlled rates so the drying occurs from the inside of the coating toward the outside of the coating. The coating dries without the formation of moisture barriers that lead to blistering. Consequently, the cellulosic electrode coating can have moisture contents higher than possible when using conventional convection drying procedures.
In accordance with the present invention, there is provided a device for drying the outer coating of a stick electrode in the form of a generally straight center welding wire of magnetic metal with an axis and surrounded by extruded, pliant mass with a settable binder. In practice, the binder is normally sodium silicate and the pliant mass includes particles such as particles of a flux system, particles of an arc stabilizer or alloying particles. The novel device comprises a conveyor to transport a succession of the electrodes along a given path, a multi-turn induction heating coil extending along, and spaced from, the moving electrode and a power source for passing an alternating current through the coil to induce alternating voltage differences in the welding wire to cause AC current to flow in the wire to heat the wire. In this manner, the coating is heated from the wire toward the outside, instead of from the outside toward the wire. In some embodiments of the invention, the coil encircles the path and defines a passageway for the moving electrodes. In other embodiments, the coil is on one side of the path or a coil section is on each side of the path. In the preferred embodiment, the electrodes are transported transversely in side-by-side relationship and, thus, the path is perpendicular to the axes of the moving electrodes. In accordance with an aspect of the invention, the power source has an output frequency in the general range of 50-5,000 hertz and a power of over 5 kW total, from all of the induction heater power sources combined.
In accordance with an aspect of the invention, a convection or radiant heater surrounds the moving path of the electrodes to heat the coating by convection or radiation. The convection or radiation heating device can be before the induction heating coil, after the induction heating coil or coextensive with the induction heating coil. Thus, the convection or radiation heating is supplemental to heating of the wire by induction heating. The spacing or gap between the coil and the center wire is selected using standard induction heating technology; however, it is generally less than about 1.0 inches. This gap controls heating created by the flux field caused by the alternating current driving the coils. The field induces a voltage causing heating current in the wire. In practice, the coil has at least four turns and the wire passageway is at least 16 inches wide. If the invention is performed by a pancake type induction heating coil with the coil on one side of the path, the coil has a transverse width of at least about 16 inches. The wire “passageway” is merely a location below or above the pancake induction heating coil. The induction heating coil is driven by a power source that delivers at least about 5 kW to the coil. 10 kW minimum power level is reasonable for all but cellulosic electrodes. The minimum power is not a feature of the invention and involves all power from the induction heater power source combined. The residence time associated with induction heating is controlled by the speed of the electrodes moving through or past the induction heating coil.
In accordance with still a further aspect of the invention, there are two induction heating coils extending along the path. These coils are at least partially coterminous. A first power source drives the first coil and a second power source drives the second coil, so one of the induction heating coils is spaced from the wire and has a frequency and power to inductively heat the wire. The second coil surrounds the moving electrode and has a frequency and power for inducing current flow in the individual magnetic particles of the coating. Using this aspect of the invention, the wire itself is heated inductively and causes conduction heating of the outer coating. At the same time, each of the individual alloying particles is inductively heated to provide an internal heating source for the coating itself. The heating is still from the inside toward the outside to prevent a skin being developed on the outer surface of the moving electrodes.
Another aspect of the present invention is the provision of a method of at least partially drying the outer coating of a stick electrode in the form of a generally straight welding wire of magnetic metal with an axis and surrounded by an extruded, pliable mass with a settable binder. The invention involves moving the electrodes in succession along a given path, inductively heating the wire with a current having a given frequency and continuing the heating until the coating is generally hardened by heating from the inside out. This method is performed by the device forming the primary aspects of the present invention. The term “dried” or “drying” as it relates to induction heating includes the concept of partially drying.
The primary object of the present invention is the provision of a device and method for drying the outer coating of a stick electrode, which device and method inductively heats the center wire for heating the coating from the inside out to dry the coating.
Another object of the present invention is the provision of a device and method, as defined above, which device and method allows control of the moisture in the coating and prevents minute cracks in the coating during the drying process.
Still a further object of the present invention is the provision of a device and method, as defined above, which device and method allows control of the rate and time of heating and does not produce an outer skin, especially on cellulose type coatings.
A further object of the present invention is the provision of an apparatus and method, as defined above, which apparatus and method controls the moisture of the coating, is repetitive, is clean, is efficient and is environmentally friendly.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.